Diamond core drilling utilises an annular drill bit connected to a core barrel assembly. The core barrel is attached to the end of a number of tubular drill rods connected to form a drill string. The drilling progressively removes cylindrical cores of rock or material through which the drill and drill tube advance using a sequence of runs. This type of drilling utilises an inner tube assembly which has an inner tube connected to a head assembly to receive the core sample. The head assembly comprises a latch body connected to a valve housing which in turn is connected to a bearing housing which in turn is connected to an inner tube connector. The inner tube assembly connects to the inner tube connector and may comprise an inner tube, core lifter and core lift case. The inner tube assembly locates within a core barrel which comprises a combination of drill bit, reamer, outer tube, landing ring and locking coupling. The inner tube assembly can be retrieved from the surface when the inner tube is full. Empty inner tube assemblies can be delivered from the surface to the bottom of the drill string in order to recommence drilling.
The drill bit is advanced by rotating the drill string while applying downward pressure. In addition, drilling fluid such as water or drilling muds are pumped through the centre of the drill string, past the inner tube assembly and through the end of the drill bit in order to carry cuttings and other drilling debris to the surface via the annulus between the wall of the hole and the external surface of the drill string.
The hole being drilled may range from vertical, angled downwardly, horizontal, inclined upwardly or directly upwardly. The holes being drilled may be either normal or dry. In dry holes, the drilling fluid drains away or partially drains away naturally through crevasses or other openings in the rock strata through which the drill passes. In normal holes, the drilling fluid does not drain away.
Hereafter, normal holes, which retain water or partially retain water will be referred to as wet holes.
Of course, in the case of horizontal or near horizontal holes, it is likely that drilling fluid would naturally drain away particularly when the inner tube assembly is being retrieved or after the inner tube assembly is pumped back into the end of the drill string.
With either locating an inner tube assembly within a drill string or retrieving an inner tube assembly, when the inner tube is full, it is most often the case that an overshot tool is used to either lower the inner tube assembly into place, be used to pump the inner tube assembly into place or be lowered by itself to retrieve the inner tube assembly. In the case of a dry hole, the overshot tool will hold the inner tube assembly and their combined weight will allow the inner tube assembly to be lowered into position. In the case of a wet hole, or where the hole is partially wet or where the hole is horizontal, inclined upwardly or upwardly vertical, then the overshot tool has a sealing section which is fluid tight and enables fluid pressure to push the overshot tool and the attached inner tube assembly along the drill tubing.
The overshot tool, when used to retrieve an inner tube assembly when the inner tube assembly is full, can either fall under gravity to latch with the core barrel assembly or again be pumped into place to latch with the core barrel assembly.
A conventional latch in an inner tube assembly of a smaller diameter comprises a pair of pivoted and opposed arms. The lower portions below the pivot point of the arms have a resilient or biasing means which draw the lower ends together. This in turn causes the upper ends of the latch members to project outwardly from the inner tube assembly. In this position they can engage with a locking coupling included in the core barrel to latch the inner tube assembly with respect to the core barrel.
The overshot tool needs to cooperate with this latch so that the inner tube assembly can be held with respect to the overshot tool when it is being placed into position within the drill string and core barrel or alternatively must cooperate with the latch when the overshot tool is being used to retrieve the inner tube assembly.
In the case of retrieval, the overshot tool needs to engage the latch so that it releases the inner core assembly from the core barrel while at the same time the latch also needs to engage the overshot tool so that the overshot tool is held by the core barrel latch which allows for withdrawal of the inner tube assembly from the drill tube.
Once the retrieved inner tube assembly is at the surface, the latch needs to be easily disengaged from the overshot tool.
Various means exist for performing all of the above functions. However, it is the case that there are a number of different pieces of equipment that are used depending on the type of hole being drilled. The hole may be wet or dry and it may be vertically down or vertically up or any inclination in between. This means that the set of equipment, being the inner tube assembly and overshot tools differ in their configuration and operation depending on the type of hole being drilled.
It is against this background and the properties and difficulties associated therewith that the present invention has been developed.
Certain objects and advantages of the present invention will become apparent from the following description taking in connection with the accompanying drawings, wherein, by way of illustration and example, and embodiment of the present invention it is disclosed.